1. Field of the Invention
The present invention relates to a wireless transmission system, such as an analog cordless telephone system, wherein a parent device and one or more child devices communicate a variety of different data types using wireless transmission.
2. Description of the Related Art
Analog cordless telephone systems, including a parent device and at least one child device, have recently become popular. Audio signals are transmitted between the parent device and the child devices over a free channel selected from a plurality of channels. The audio signals are frequency modulated (FM) and transmitted using a full duplex transmission scheme.
The parent and child devices will need to transmit and receive data other than the audio data of the telephone call, before, during, and after a telephone call actually takes place. For example, when the user attempts to make a telephone call using a child device, the child device must transmit the dialing information to the parent device. Also, the parent device will sometimes need to transmit information to the child device about whether the telephone line is connected. In cases like this, the selected channel is used to communicate data packets using minimum shift keying (MSK).
Data packets are normally transmitted and received in half duplex mode, for example because of limitations in processing ability of the CPU. Here, an explanation will be provided for half duplex operations performed when a parent device needs to transmit a data packet to a child device. First, the parent device confirms that it is not itself receiving a data packet. After confirming this, the parent device transmits its own data packet to the child device. When the child device receives the data packet from the parent device, it sends a response packet to the parent device in return. If the parent device does not receive such a response packet from the child device, even after a fixed retry waiting time elapses from when the parent device initially transmits the data packet, then the parent device retransmits the data packet. In this way, transmission and reception of data can be reliably performed.
However, in an analog cordless telephone system, there is no way for the parent and child devices to predict when the other device will transmit a data packet. Therefore, there will be times when both the parent and child devices attempt to transmit data packets at the same time. In this case, the data packets transmitted from the parent and child devices will collide with each other.
Because data packets that collide can not be properly received, neither device will transmit a response packet. When no response packet is received, both devices will attempt to retransmit their data packets. If both the parent and child devices are set with the same retry waiting time, then the resent data packets will again collide with each other. To avoid this problem, the parent and child device can be designed to randomly change the retry waiting time.
However, if the retry waiting time is random, then it will be impossible to predict which of the parent or the child device will first receive the resent data packets after a collision. Also, time required to retransmit the data packets fluctuates. Further, the possibility remains that the data packets will again collide with each other. As a result, data transmission and reception is unstable.
It is an objective of the present invention to provide a wireless transmission system and a wireless transmission method for communicating data between parent and child devices in half duplex mode, wherein communication of data is easily controlled, and data packets are properly resent after data packets collide.
To achieve the above-described objectives, a wireless transmission system according to the present invention includes a parent device and at least one child device configured in the following manner. The parent device and the at least one child device establish a wireless connection to exchange data packets using half duplex transmission. The parent device and the at least one child device resend a data packet for which no response packet was received. One of the parent device and the at least one child device wait a retry waiting time Tb1 from transmission of the data packet before resending a data packet, and the other of the parent device and the at least one child device wait a retry waiting time Tb2 from transition of the data packet before resending a data packet. The retry waiting time Tb1 is set to satisfy the following relationship with the retry waiting time Tb2, a propagation delay Td from when a data packet is sent until the data packet is supposed to reach its intended destination, and a time width Tp of the data packet:
Tb1xe2x89xa7Tb2+2Td+Tp.
That is, both the parent device and the child device retransmit data packets when they do not receive a response packet for a transmitted data packet after a retry waiting time elapses from when the original data packet was transmitted. However, the parent and the child devices use different retry waiting times Tb1. and Tb2 to determine if and when to retransmit a data packet. One retry waiting time Tb1 is greater than or equal to the other retry waiting time Tb2, plus two times a propagation delay Td from when the data packet is sent until the data packet is received, plus a time width Tp of the data packet, that is, Tb1xe2x89xa7Tb2+2Td+Tp. With this configuration, even if data packets transmitted from the parent and the child device collide with each other, one of the data packets will be reliably transmitted before the other, so that stable reception and transmission of data can be performed.
The parent device can be set to use the retry waiting time Tb1 and the at least one child device can be set to use the retry waiting time Tb2. In this case, because the parent device is set with the retry waiting time Tb1, which is equal to or greater than the sum of Tb2+2Td+Tp, when the transmission of data packets from the parent and child devices collide with each other, the data packet from the child device will be retransmitted with priority over the data packet from the parent device. This configuration is particularly suitable for use in a cordless telephone system, wherein the child device frequently transmits data such as dialing information to the parent device.
Alternatively, the parent device can be set to use the retry waiting time Tb2 and the at least one child device can be set to use the retry waiting time Tb1. Because the child device is set the retry waiting time Tb1, which is equal to or greater than the sum of Tb2+2Td+Tp, when transmission of data packets from the parent device and the child device collide with each other, the data packet from the parent device will be resent with priority over the data packet from the child device. This configuration is particularly well suited for situations where it is desired to reliably transmit extremely important data from the parent device, such as information that tells the child device whether or not the telephone circuit is connected.
It is desirable that the parent device and the at least one child device be adapted to receive a priority command indicating that one of the parent device and the child device has priority for exchange of data packets. When the priority command indicates the parent device has priority, then the parent device uses the retry waiting time Tb2 and the at least one child device uses the retry waiting time Tb1. On the other hand, when the priority command indicates the at least one child device has priority, then the parent device uses the retry waiting time Tb1 and the at least one child device uses the retry waiting time Tb2.
With this configuration, a priority command can be transmitted between the parent device and the child device, to designate which of the parent and child devices is to be given priority. The device designated as having priority is set with the retry waiting time Tb2, and the device not designated as having priority is set with the retry waiting time Tb1, wherein Tb1xe2x89xa7Tb2+2Td+Tp as mentioned above. Accordingly, reception and transmission of data is stable and also flexible. When transmission of data packets from the parent and child devices collide, the data packet that needs to be resent first can be transmitted with priority as best suited for the present circumstances.
It is desirable that the wireless transmission system is a cordless telephone system connected to a telephone line. When the present invention is applied to a cordless transmission system, the retry waiting times for communication of data between the parent and the child devices are controlled as described above. Accordingly, when necessary information, such as dialing information and line connection information, is transmitted over a transmission channel using a half duplex transmission scheme, reception and transmission of data can be stably performed using a simple control configuration that does not increase costs.